In general, a Ni--MH secondary battery in which a hydrogen-absorbing alloy is employed as an anode material, works on a reaction principle as followings: In the course of electric: discharge of a battery, hydrogen atoms within the hydrogen-absorbing alloy bind to OH.sup.- ions of KOH electrolyte to give water, and electrons simultaneously move to a cathode through external circuit. During electric charging, water is electrolysed to give H.sup.+ and OH.sup.- ions, where OH.sup.- ions stay in the electrolyte, and H.sup.+ ions bind to influxed electrons to give hydrogen atoms, which, in turn, bind to the hydrogen-absorbing alloy finally to be stored within the alloy. This reaction occurred, based on the properties of hydrogen-absorbing alloy that it is stable in an alkaline solution and absorbs/releases a lot of hydrogen rapidly and reversibly.
In order to use the hydrogen-absorbing alloy system as an anode material of a Ni--MH secondary battery, the alloy system should meet following requirements: First, the hydrogen-absorbing alloy has to possess hydrogenation properties such as proper hydrogen absorption-desorption pressure in a solid-gas reaction (generally, 0.01 to 1 atmosphere at room temperature), rapid hydrogenation rate, and a high hydrogen-absorbing capacity(a theoretical discharge capacity of an electrode is proportional to a hydrogen-absorbing capacity (C.sub.H (wt. %)): a theoretical discharge capacity (mAh/g)=268.times.C.sub.H)). Secondly, charge transfer associated with the oxidation and reduction of hydrogen at the interface between the alloy and the KOH electrolyte, has to occur easily during the electrochemical reaction of the alloy and the electrolyte. Accordingly, only the hydrogen-absorbing alloy whose surface functions as a catalyst for charge transfer reaction can be used as an anode material of a Ni--MH secondary battery.
So far, many hydrogen-absorbing alloy systems satisfying the said requirements have been reported, whose examples includes: AB.sub.5 type-hexagonal structure of La--Nd--Ni--Co--Al alloy system (see: U.S. Pat. No. 4,488,817), Mm--Mn--Ni--Co--Al alloy system (see: JP 61-1132501; JP 61-214361); Ti--V--Ni--Cr alloy system of AB.sub.2 type-C14,15-hexagonal and BCC (body-centered cubic lattice) multiphase structure (see: U.S. Pat. No. 4,551,400); Zr--V--Ni alloy system of C14 structure (see: J. of the Less-Common Metals, 172-174:1219 (1991)); and, Zr--Cr--Mn--Ni alloy system of C14, C15 structure (see: J. of the Less-Common Metals, 172-174:1211(1991)).
Among the said alloy systems, La-Ni electrode of AB.sub.5 type shows highly reduced capacity during charge/discharge cycling in an alkaline electrolyte (see: J. of the Less-Common Metals, 161:193(1990); J. of the Less-Common Metals, 155:119(1989)), which is called as "degradation". J. J. G. Willems et al. have substituted a small amount of Ni with Co, Al, and a small amount of La with Nd to increase durability for charge/discharge cycling, while it results in the reduction of capacity (see: U.S. Pat. No. 4,488,817).
On the contrary, it has been reported that electroless hydrogen-absorbing alloy powder with copper results in an increase in the durability for charge/discharge cycling without the reduction of capacity (see: J. of the Less-Common Metals, 107:105(1985)). However, the said method essentially accompanies a step of plating and environmental pollution caused by solutions employed therein.
On the other hand, it has been found that hydrogen-absorbing alloy of AB.sub.2 type has a discharge capacity of 300 mAh/g or more, which is higher than that of AB.sub.5 type alloy, and has a good durability for charge/discharge cycling without a step of plating (see: J. of the Less-Common Metals, 172-174:1175(1991); J. of the Less-Common Metals, 180:37(1992)).
In addition, T. Gamo et al. discloses a hydrogen-absorbing Z-based alloy system which comprises Zr over 30 at. % and Ni over 40 at. % and has a discharge capacity of 300 to 370 mAh/g (see: U.S. Pat. No. 4,946,646). Also, K. Hong and M. A. Fechenko et al. teaches a hydrogen-absorbing alloy system of Ti--Zr--V--Ni--Cu--Mn--M (M.dbd.Al, Co, Fe, etc.) having a discharge capacity of 300 to 380 mAh/g (see: U.S. Pat. No. 4,849,205; U.S. Pat. No. 4,728,586; U.S. Pat. No. 4,551,400).
All of the said conventional hydrogen-absorbing alloys are, however, proven to be less satisfactory in the sense that they have discharge capacities of 250 to 320 mAh/g (AB.sub.5 type) and 300 to 380 mAh/g (AB.sub.2 type), which are lower than 400 mAh/g.
Recently, in accordance with the advent of electric vehicles and electronic machines such as cellular phone, notebook computer and camcorder, etc., and tendency of miniaturization of the electronic machines, there are strong reasons for exploring and developing alternative batteries of high capacity. However, a hydrogen-absorbing capacity of AB.sub.5 type alloy can not reach to 1.2 wt % (a discharge capacity of 320 mAh/g) or more, since the alloy has a molar ratio of A.sub.1/6 B.sub.5/6 and a molar molecular weight of 72 g/mol. Also, a hydrogen-absorbing capacity of Z-based alloy system, among the AB.sub.2 type alloys, can not reach to 1.4 wt % (a discharge capacity of 400 mAh/g) or more, since the alloy has a molar ratio of A.sub.1/3 B.sub.2/3 and a molar molecular weight of 67 g/mol. Therefore, needs have continued to exist for the development of a new hydrogen-absorbing alloy having a discharge capacity over 400 mAh/g, since further increase in the energy density of a battery can not be expected in the conventional hydrogen-absorbing alloys.
Accordingly, the hydrogen-absorbing alloy of light weight has to be used in order to down the molar molecular weight of the hydrogen-absorbing alloy to be under 65 g/mol, whose examples include alloy systems comprising Mg, V and Ti elements. However, the Mg alloy system can not be used for an electrode, since it has low hydrogen-absorption/desorption pressure at room temperature and slow hydrogenation rate. Although the V alloy system, which is substituted with a small amount of Ti and Zr, has proper hydrogen-absorption/desorption pressure of 0.01-1 atm at room temperature, it can not be used for an electrode, since it can not function as a catalyst of charge transfer reaction.
On the other hand, the hydrogen-absorbing Ti alloy system has been evaluated as a proper hydrogen-absorbing material, since it has a large hydrogen-absorbing amount(about 1.96 wt % H.sub.2 /alloy(g)) and high reaction rate. In this connection, T. Gamo suggests a hydrogen- absorbing Ti alloy system (see: U.S. Pat. No. 4,144,103; U.S. Pat. No. 4,160,014), however, it has a relatively high hydrogen equilibrium pressure of 5 to 10 atm and whose surfaces can not function as catalysts of charge transfer reaction in KOH electrolyte. Therefore, the alloy system can not be practically applied as an anode material of a secondary battery.